A Principal is one that knowingly and voluntarily gets involved in a criminal offense at or before its commission.
This would include the individual that actually robs the bank as well as any individual that assisted him in doing so (such as the get away driver). All such persons are deemed “Principals” and can be convicted of the offense (i.e. the get away driver is guilty of bank robbery even though he never entered the bank). The prosecution often times accuses one of committing a crime or being an “aider and abettor” in its commission. This simply means that the person is accused in the alternative or either committing the offense themselves or assisting another in committing the offense. Both are deemed “principals” and looked upon by the law as equally responsible.
An “Accessory” is one that gets involved in a criminal offense after its commission. This accusation means that the individual is not a “Principal” as discussed above; however, did somehow get involved after the crime had been committed by another. Generally, this accusation means that the person did something to try and cover up or hide the fact that the crime was committed or harbored and protected the person they believe did commit the offense.